[chinoyenmose]

Closes #1543
Closes #1544
Closes #1545
Closes #1546

What changed

• [Frontend] Implement WCAG 2.1 AA accessible focus trap and ARIA attributes in Density Grid #1543: WCAG 2.1 AA — role="grid"/role="gridcell" semantics, aria-label/aria-selected, focus trap in detail drawer, aria-modal dialog
• [Frontend] Add responsive mobile drawer and touch gestures to Density Grid #1544: Responsive mobile drawer (bottom sheet on mobile, centered modal on desktop) with swipe-to-close and horizontal swipe to navigate columns
• [Frontend] Implement optimistic state rollback on network failure in Density Grid #1545: Optimistic state rollback — useOptimisticData hook retains last known good snapshot when a refetch returns empty/invalid data, preventing grid wipe
• [Frontend] Enhance keyboard navigation and shortcut bindings for Density Grid #1546: Keyboard navigation — Arrow keys, Home/End, PageUp/PageDown to move between cells; Enter/Space to open details; Escape to close; roving tabIndex pattern

Why

• The heatmap was a static SVG with no keyboard access, no ARIA semantics, no mobile-friendly detail view, and no resilience to failed data refetches

How to test

• Tab to the grid, use arrow keys to navigate cells, Enter to open detail drawer
• Focus is trapped in the drawer; Escape closes it and returns focus to the grid
• On mobile, tap a cell to open the bottom sheet; swipe down to dismiss
• Horizontal swipe on the grid navigates columns
• npx vitest run — 665 passed (identical to clean tree; 108 pre-existing failures unrelated)

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